Regulation of Stroke Volume
Ventricular stroke volume (SV) is the difference between the ventricular end-diastolic volume (EDV) and the end-systolic volume (ESV). The EDV is the filled volume of the ventricle prior to contraction and the ESV is the residual volume of blood remaining in the ventricle after ejection. In a typical heart, the EDV is about 120 ml of blood and the ESV about 50 ml of blood. The difference in these two volumes, 70 ml, represents the SV. Therefore, any factor that alters either the EDV or the ESV will change SV.
SV = EDV - ESV
For example, an increase in EDV increases SV, whereas an increase in ESV decreases SV.
There are three primary mechanisms that regulate EDV and ESV, and therefore SV.
Afterload is related to the pressure that the ventricle must generate in order to eject blood into the aorta. Changes in afterload affect the ability of the ventricle to eject blood and thereby alter ESV and SV. For example, an increase in afterload (e.g., increased aortic pressure) decreases SV, and causes ESV to increase. Conversely, a decrease in afterload augments SV and decreases ESV. It is important to note, however, that the SV in a normal, non-diseased ventricle is not strongly influenced by afterload. In contrast, the SV of hearts that are failing are very sensitive to changes in afterload.
Changes in ventricular inotropy (contractility) alter the rate of ventricular pressure development, thereby affecting ESV and SV. For example, an increase in inotropy (e.g., produced by sympathetic activation of the heart) increases SV and decreases ESV. Conversely, a decrease in inotropy (e.g., heart failure) reduces SV and increases ESV.
It is important to note that the effects of changes in EDV and ESV on SV are not independent. For example, an increase in ESV usually results in a compensatory increase in EDV. Furthermore, if SV is increased by increasing EDV, this can lead to a small increase in ESV because of the influence of increased afterload on ESV caused by an increase in aortic pressure. Therefore, while the primary effect of a change in preload, afterload or inotropy may be on either EDV or ESV, secondary changes can occur that can partially compensate for the initial change in SV. For a more detailed description of these interactions, see the web pages describing preload, afterload, or inotropy.